Jessica Chen said she and dozens of others barricaded themselves in a conference room on the second floor of 345 Park Ave. after hearing shots fired in the building.

Officials said the gunman fatally shot himself after his attack, which set off a chaotic scene that rippled out into the heart of Manhattan during rush hour.

Investigators are combing through evidence in the wake of New York City’s deadliest mass shooting since 2000, trying to figure out more about the gunman who unleashed an assault-style rifle in Midtown,